U0126 effectively and specifically inactivates Mos–MAPK in starfish oocytes and alters spindle morphology. (A) Oocytes were treated with the indicated concentrations of U0126 concomitant with maturation hormone addition. Thereafter, maturation was monitored by videomicroscopy, of which selected frames are shown at the start of the recording and 60 min after hormone addition. (B) Western blots of oocytes treated as in A using phospho-MAPK and phospho-Cdk1 antibodies to monitor Mos–MAPK and Cdk1 activities, respectively. Numbers to the left are in kDa and mark the position of the molecular weight markers. For the complete blots, see Source Data FS1. (C) Immunofluorescence images of oocytes stained for microtubules (anti-tubulin, cyan) and chromatin (Hoechst 33342, red) at the indicated stages and treated with 20 µM U0126 or an equal amount of DMSO. Imaging was done on a laser scanning confocal microscope. Maximum intensity projections are shown; scale bars are 20 µm. (D) Schematics of the meiotic spindle illustrating the morphometric measurements. Microtubules are cyan, and chromatin is red. (E) Individual data points and boxplots showing 3D measurements performed on images similar to those shown in C. **P < 0.05, ***P < 0.0001; statistical analysis was done using a two-tailed t test. n is the number of oocytes analyzed. Source data are available for this figure: SourceData FS1.